1. Field of the Invention
The present invention relates to a sliding component that is used in, for example, a mechanical seal.
2. Description of the Related Art
A sealing device (for example, mechanical seal) including relatively rotating two components in which planar end faces slide on each other has been known as a sealing device that prevents (suppresses) leakage of sealed fluid. In such sealing devices, in order to maintain a good sealed state for long periods, it is necessary that a fluid lubricating film be formed between sliding surfaces of the two components by the sealed fluid. Therefore, abrasion or roughened surface can be prevented (suppressed). In order to maintain a sealing property for long periods, it is necessary to balance contradictory conditions of “sealing” and “lubrication”.
Recently, for the purpose of environmental measures, a demand for low friction arises in order that a mechanical loss is reduced while the leakage of the sealed fluid is prevented.
When the sliding surfaces of the two components are formed with a smooth surface like a general mechanical seal, it has been known that not only a film of the sealed fluid but also a phase by cavitation generated in the fluid are formed between the sliding surfaces (see Hamilton, Walowit, Allen: A. S. M. E. Paper No. 65-Lub-11 (1965)). That is, a phase (hereinafter referred to as liquid phase) made of the liquid (sealed fluid) and a phase (hereinafter referred to as gas phase) made of gas are formed between the sliding surfaces.
It is confirmed that the phases formed between the sliding surfaces express an extremely unstable behavior even if the relative rotation of the two components is steady. The cavitation that is of a factor of forming the gas phase is generated by minute irregularity of the sliding surface. The cavitation is generated in a random position, and the behavior of the cavitation is unstable after the generation. Therefore, the behavior of the liquid phase and the gas phase become unstable. The unstable behavior of the two phases will be described with reference to FIGS. 21A to 21D. FIGS. 21A to 21D illustrate an example of the behavior of the liquid phase and the gas phase that are produced between the sliding surfaces of the two components that rotate relatively at a constant speed.
In FIGS. 21A to 21D, the meshed portions indicate liquid phases, and the portions that are not meshed indicate gas phases. In FIGS. 21A to 21D illustrate different timing states while the two components rotate relatively at a constant speed (one of the two components rotate while the other stands still).
As can be seen from FIGS. 21A to 21D, the mixed state of the liquid phase and the gas phase changes complicated with time. It is also confirmed an increase in friction coefficient in which the sliding surfaces comes into direct contact with each other while the whole sliding surface is instantaneously covered with the gas phase. The phenomenon is attributed to the fact that the sliding surface formed with the smooth surface does not have a mechanism or a structure that retains and stabilizes the film (phase) made of the liquid between the sliding surfaces.
There has been also known a sliding component in which micro holes or dimples are made on the sliding surface to improve lubricating property (see Japanese Patent No. 3026252).
FIG. 22 is a plan view partially illustrating a sliding surface of a sliding component according to a conventional example. As illustrated in FIG. 22, a plurality of dimples 501 is provided in a sliding surface of a sliding component 500. A hydrodynamic lubricating film (film formed with the sealed fluid) is formed according to the shapes and array of the dimples 501. A lubricating liquid (sealed fluid) is supplied from the dimple 501. Conventionally, the film of the lubricating liquid (sealed fluid) is stably formed in the sliding surface by providing the dimples 501 in the sliding surface of the sliding component 500, thereby aiming to improve the suppressing effect of the sliding abrasion.
However, the sealing performance is degraded when the sealed fluid is supplied to the sliding surface by the micro holes such as dimples to encourage film formation of the sealed fluid. Therefore, there is a risk of losing the original function of the seal. In order to maintain the sealing performance, it is necessary to block out a leakage direction, that is, a flow of the sealed fluid in a radial direction of the sliding surface. However, the conventional sliding component does not include the mechanism or structure that blocks out the flow of the sealed fluid in the radial direction.